Extremely high cancer incidence in patients of Fanconi Anemia (FA, a rare human genetic disease) has long suggested that the FA signaling pathway is a tumor suppressor pathway. However, it remains poorly understood as to how this tumor suppressor pathway functions in human tumorigenesis. Our long-term goal is to use FA as a unique genetic model system to dissect the FA signaling pathway, determine how FA proteins mediate tumor suppression, and investigate the potential of targeting the FA pathway as a therapeutic approach in cancer treatment. Our findings, for the first time, indicated that the FA pathway plays a significant role in suppressing the development of cancer in patients without FA. Studies on FA group D2 protein (FANCD2), the central controller of the FA signaling pathway, have made a huge contribution to our understanding of roles of this pathway in cancer development as well as cancer treatment. We have reported that, upon DNA damage, activated FANCD2 is a nodal point between the FA and HHR6 (human homolog of yeast Rad6) pathways in maintaining genomic stability. We have also reported that inactivated FANCD2 gains a new role in upregulating the oncogene Np63 to promote tumor development, in addition to losing the roles performed by activated FANCDD2 in guarding genome integrity. Unexpectedly, we found a new variant of FANCD2 named FANCD2-V2 whose primary sequence is 20 amino acids longer than that of the known variant of FANCD2, now named FANCD2-V1. Both V1 and V2 had previously been presumed to be one protein owing to limitations of detection methods. Therefore, many of the reported functions of FANCD2 could reflect the properties of both V1 and V2, rather than FANCD2 (now V1) only. However, the functions of FANCD2-V2, unlike V1, remain totally unclear. Our unpublished data show that the high expression ratio of FANCD2-V2 over V1 is significantly associated with lower tumor stages (r=-0.206, p=0.004) or grades (r=-0.226, p=0.0027) in human ovarian cancer (n=190), and that ectopic expression of FANCD2-V2 can suppress tumor cell growth in vitro and in vivo. On the basis of our published and unpublished data, we have formulated central hypothesis that FANCD2-V2 is a potent tumor suppressor that protects human cells from neoplastic transformation. Specifically, we will address molecular mechanisms underlying the roles of activated FANCD2-V2 & inactivated FANCD2-V2 in the specific aim one and two to test the 1st part of the central hypothesis. We will also address biological properties of this overlooked tumor suppressor in the specific aim three to test the 2nd part of the central hypothesis. Many years' studies of FANCD2 have overlooked FANCD2-V2. Our proposed experiments are the first to systematically characterize FANCD2-V2. These studies will 1) advance our understanding of how genomic stability is maintained by the FA pathway, 2) elucidate the complexity in the pathogenesis of human cancer, and 3) explore the utility of FANCD2-V2 as prognostic or predictive biomarker. This includes promises in the immediate development of more effective tools for the fight against human ovarian cancer.